In the field of fabricating VLSI circuits the metallization materials that are used for establishing contacts and interconnects are known to cause failures due to electromigration or other interaction at the interface between the metallization material and the substrate, or between successive metallization materials. To reduce the likelihood of the occurrence of these failures a diffusion barrier is introduced between the metallization material and the substrate, or between two successive metallization materials.
Titanium-tungsten, for instance, has been widely used as a diffusion barrier between aluminum-based films and a silicon substrate. It has become known that relatively poor vacuum levels during sputtering of titanium-tungsten improve the quality of the barrier integrity. It has also been recognized that an interfacial oxide layer between the aluminum-based film and the titanium-tungsten film is needed for good barrier properties. The oxide formed by oxidizing titanium-tungsten is found to be a mixture of titanium oxide and tungsten oxide. See, for instance, H. G. Tompkins et al., "An investigation of the oxidation of Ti:W", J. Appl. Phys. 64(6), 15 September 1988, pp. 3269-3272. In older systems, the titanium-tungsten film is deposited on the wafer by sputtering, carried out in a first bell jar system, after which the bell jar is opened and the wafer is transferred to another system for the deposition of aluminum. This procedure exposes the wafer to air, thereby oxidizing the wafer's surface and furnishing a native oxide over the titanium-tungsten film. The opening of the bell jar also exposes the titanium-tungsten target to air, thereby oxidizing its surface. This results in a less pure film of titanium-tungsten in the next deposition cycle.
In modern systems, such as load-lock systems, wafers are kept under a higher vacuum during titanium-tungsten deposition than is attainable in older systems. As a result, the titanium-tungsten films thus deposited are purer than those formed in the more conventional systems. Purer titanium-tungsten films turn out to have relatively poor barrier performance. In addition, the load-lock systems are generally used to perform a series of different deposition steps without removing the wafer and, as a consequence, without breaking the vacuum. This excludes the growth of an oxide on the targets.
Alternatively, owing to general improvements in manufacturing cycle times, the time span between the titanium-tungsten deposition and the aluminum deposition is reduced considerably. The shorter sit times after titanium-tungsten deposition lead to poor barrier performance since the oxide layer on top of the titanium-tungsten by the exposure to air is inadequate, if formed at all.
These barrier failures become apparent particularly by low forward voltages of Schottky diodes that are fabricated at the contact level to the silicon semiconductor substrate.
The barrier properties become worse when the titanium-tungsten is subjected to further heat treatments at high temperatures as in modern processes. For example, the titanium-tungsten that is employed at the lowest metal level currently used must withstand the following heat treatments: spin-on-glass cures up to 480.degree. C. for the contact dielectric and again for the via dielectric; chemical vapor deposition or plasma enhanced deposition of oxides and nitrides at temperatures up to 450.degree. C. for the contact, via and passivation dielectrics; and heat treatments, typically at 450.degree. C., to anneal oxide defects.
In short, use of modern fabrication equipment and the attendant cycle time reduction lead to poorer barrier layers.
One obvious solution to alleviate the problem of poor barrier performance would be to deliberately expose the titanium-tungsten to air at ambient temperature. However, exposing the titanium-tungsten film to air for even 4 hours prior to aluminum deposition does not bring about the required barrier quality. Furthermore, an additional process step of this length is unacceptable in cost-effective fabrication.
Other solutions would be to include exposing the barrier metal to an oxidizing ambient at higher temperatures, for instance at about 200.degree. C. or higher, or using plasma assisted oxidation. However, there is a delicate balance between sufficient oxidation and too much. The required thickness of the oxide layer is thought to lie in the range of 10-20 .ANG.. A layer thinner than 10 .ANG. would not attain the desired barrier properties, whereas a layer thicker than 20 .ANG. would lead to etching difficulties later on and to increased contact resistance. Oxidation at higher temperatures has the disadvantage that the oxide layer grows too rapidly, thereby rendering its thickness difficult to control. See FIG. 4 of H. G. Tompkins et al., (ibid.). Oxidation in a plasma oxygen ambient has been reported to give an oxide thickness of about 40 .ANG. in 5 minutes. See J. -S. Maa et al., "Reflectivity reduction by oxygen plasma treatment of capped metallization layer", J. Vac. Sci. Technol. B7(2), March/April 1989, pp. 145-149. It can be inferred that the time necessary to obtain a film of 10-20 .ANG. should lie approximately between 1-2 minutes, possibly depending on the sit time before the plasma oxidation. Therefore, control of the oxide thickness would be difficult, unless the oxidation is performed in-situ in the titanium-tungsten deposition system without breaking the vacuum. However, a system designed for both titanium-tungsten deposition and plasma oxidation is not commercially available at this time.